This invention relates generally to the field of analog to digital conversion and more specifically to high performance analog-to-digital conversion using a linearized sampler.
An analog to digital (A/D) converter is an important element in systems that use digital analysis or digital control of analog inputs. The A/D converter may be part of a large analog system, and is frequently the component that limits the performance of the system. Two specific performance parameters of A/D converters are resolution and sampling rate. Higher resolution A/D converters typically require a large signal to noise ratio and good linearity. A/D converters with high sampling rates are frequently desired, but generally have lower resolution. Therefore, a tradeoff is usually made between sampling rate and resolution.
Analog to digital conversion of a continuous input signal normally occurs in two stepsxe2x80x94sampling and quantization. The sampler takes a time-varying analog input signal and converts it to a fixed voltage, current, electrical charge, or other output level. The quantizer takes the constant sampled level and compares it to the closest level from a discrete range of values called quantization levels.
Performance of high frequency analog to digital converters can be limited by timing jitter (variations in the sample taking) in an electrical sampler. However, the use of an optical modulator for sampling as compared to an electrical sampler can result in higher performance with lower timing jitter. Optical modulators used for sampling have an analog electrical input signal applied to them as well as an optical input signal. The optical input signal is modulated at a predetermined frequency to facilitate the sampling of the analog electrical input signal. Early optically sampling A/D converters used multiple modulators so that both sampling and quantization could be done optically. Other configurations used a single modulator as an optical sampler together with an electrical quantizer. The optical sampler/electrical quantizer approach has the performance advantage of optics for the sampler, but the simplicity and higher resolution of electronics for the quantizer.
Previously, the linearity of the optical sampler/electrical quantizer configuration was severely limited by nonlinearities in the optical sampler. These nonlinearities can appear in the output of the optical sampler as higher order noise. However, electrical or optical linearization techniques applied to the optical sampler can substantially reduce the effect of this nonlinearity. In the case of optical linearization, the best performing linearized optical modulators used for optical sampling can only deliver third-order linearization, with the tradeoff that second-order distortion remains in the output signal. What is needed is an optical sampler/electrical quantizer configuration in which second-order modulator distortion products can be rejected.
The invention relates to an apparatus for digitizing a narrowband analog electrical signal. The apparatus includes a linearized sampler having an analog electrical input and a sampling input; a bandpass filter, in communication with the sampler, having a center frequency of at least half of the quantization rate of a quantizer; and the quantizer in communication with the bandpass filter.
In one embodiment, the apparatus includes an optical sampler, which includes a modulated optical source in optical communication with an optical modulator, an optical to electrical converter, a bandpass filter having a center frequency of at least half of the quantization rate of a quantizer, and the quantizer.
The sampler frequency in another embodiment is chosen such that second-order distortion products generated in the sampler are separated in frequency from the sampled input band.
In another embodiment the modulated optical source is a laser. In yet another embodiment the optical to electrical converter is a photodetector. In still another embodiment, an optical isolator is in optical communication with the modulated optical source. In another embodiment, an optical amplifier is in optical communication with the modulated optical source. In yet another embodiment an optical filter is in optical communication with the optical amplifier.
In still another embodiment, the optical modulator is a Mach-Zehnder optical modulator.